The United States, Russia, China, India, Israel, and France have all developed such air defense systems.[1] In the United States, missile defense was originally the responsibility of the Army. The U.S. Missile Defense Agency has developed maritime systems and command and control that will eventually be transferred to the Navy and Air Force for operation and sustainment.

Missile defense can be divided into categories based on various characteristics: type/range of missile intercepted, the trajectory phase where the intercept occurs, and whether intercepted inside or outside the Earth's atmosphere:

The types/ranges are strategic, theater and tactical. Each entails unique requirements for intercept, and a defensive system capable of intercepting one missile type frequently cannot intercept others; however there is sometimes overlap in capability.

Targets long-range ICBMs, which travel at about 7 km/s (15,700 mph). Examples of currently active systems: Russian A-135 system which defends Moscow, and the U.S. Ground-Based Midcourse Defense system that defends the United States from missiles launched from Asia. Geographic range of strategic defense can be regional (Russian system) or national (U.S. system).

Targets medium-range missiles, which travel at about 3 km/s (6,700 mph) or less. In this context the term "theater" means the entire localized region for military operations, typically a radius of several hundred kilometers. Defense range of theater defensive systems is usually on this order. Examples of deployed or soon-to-be deployed theater missile defenses: Israeli Arrow missile, American THAAD and Russian S-400 Triumf.

Missile defense can take place either inside (endoatmospheric) or outside (exoatmospheric) the Earth's atmosphere. The trajectory of most ballistic missiles takes them inside and outside the Earth's atmosphere, and they can be intercepted either place. There are advantages and disadvantages to either intercept technique.

Some missiles such as THAAD can intercept both inside and outside the Earth's atmosphere, giving two intercept opportunities.

A NORADDistant Early Warning Line (DEW) station in western Greenland is visible in the distance beyond the snow-drifted equipment pallets in the foreground of this photograph. The DEW Line was designed to track inbound ballistic missiles.

In the 1950s and 1960s, the term meant defense against strategic (usually nuclear-armed) missiles. The technology mostly centered around detecting offensive launch events and tracking inbound ballistic missiles, but with limited ability to actually defend against the missile. The Soviet Union achieved the first nonnuclear intercept of a ballistic missile warhead by a missile at the Sary Shagan antiballistic missile defense test range on 4 March 1961.

Throughout the 1950s and 1960s, the United States Project Nike air defense program focused initially on bombers, then ballistic missiles. In the 1950s, the first United States anti-ballistic missile system was the Nike Hercules, which had a limited ability to intercept incoming ballistic missiles, although not ICBMs. This was followed by Nike Zeus, which using a nuclear warhead could intercept ICBMs. However Nike Zeus had other limitations which prevented it being deployed.[citation needed] In any case, by the early 1960s the Nike Zeus was the first anti-ballistic missile to achieve hit-to-kill (physically colliding with the incoming warhead).

The Zeus missile was enhanced, and the shorter range Sprint missile was added to the Nike defense system, then called Nike-X. The system included large powerful radars and a computer complex.

Eventually, the Nike-X program was realigned and renamed Sentinel. This program's goal was to protect major U.S. cities from a limited ICBM attack, especially focusing on China.[2] This in turn reduced tensions with the Soviet Union, which retained the offensive capability to overwhelm any U.S. defense.

The Soviet Union deployed the A-35 anti-ballistic missile system around Moscow in 1966, which also defended nearby ICBM sites. That system has been upgraded several times and is still operational. The United States announced an ABM program to protect twelve ICBM sites in 1967.

In 1967, then-Secretary of Defense Robert McNamara stated: "Let me emphasize – and I cannot do so too strongly – that our decision to go ahead with a limited ABM deployment in no way indicates that we feel an agreement with the Soviet Union on the limitation of strategic nuclear offensive and defensive forces is in any way less urgent or desirable."[3]

The SALT I talks began in 1969, and led to the Anti-Ballistic Missile Treaty in 1972, which ultimately limited the U.S. and U.S.S.R. to one defensive missile site each, with no more than 100 missiles per site.

As a result of the treaty and of technical limitations, along with public opposition to nearby nuclear-armed defensive missiles, the U.S. Sentinel program was redesignated the Safeguard Program, with the new goal of defending U.S. ICBM sites, not cities. The U.S. Safeguard system was deployed to defend the LGM-30 MinutemanICBMs near Grand Forks, North Dakota. It was deactivated in 1976 after being operational for less than four months due to a changing political climate plus concern over limited effectiveness, low strategic value, and high operational cost.[citation needed]

In the early 1980s, technology had matured to consider space based missile defense options. Precision hit-to-kill systems more reliable than the early Nike Zeus were thought possible. With these improvements, the Reagan Administration promoted the Strategic Defense Initiative, an ambitious plan to provide a comprehensive defense against an all-out ICBM attack. Reagan established the Strategic Defense Initiative Organization (SDIO), which was later changed to the Ballistic Missile Defense Organization (BMDO). In 2002, BMDO's name was changed to its current title, the Missile Defense Agency (MDA). See National Missile Defense for additional details. In the early 1990s, missile defense expanded to include tactical missile defense, as seen in the first Gulf War. Although not designed from the outset to intercept tactical missiles, upgrades gave the Patriot system a limited missile defense capability. The effectiveness of the Patriot system in disabling or destroying incoming Scuds was the subject of Congressional hearings and reports in 1992.[4]

There are still technological hurdles to an effective defense against ballistic missile attack. The United States National Ballistic Missile Defense System has come under scrutiny about its technological feasibility. Intercepting midcourse (rather than launch or reentry stage) ballistic missiles traveling at several miles per second with a "kinetic kill vehicle" has been characterized as trying to hit a bullet with a bullet. Despite this difficulty, there have been several successful test intercepts and the system was made operational in 2006, while tests and system upgrades continue.[5] Moreover, the warheads or payloads of ballistic missiles can be concealed by a number of different types of decoys. Sensors that track and target warheads aboard the kinetic kill vehicle may have trouble distinguishing the "real" warhead from the decoys, but several tests that have included decoys were successful. Nira Schwartz's and Theodore Postol's criticisms about the technical feasibility of these sensors have led to a continuing investigation of research misconduct and fraud at the Massachusetts Institute of Technology.[6]

As of February 2007, the U.S. missile defense system consists of 13 ground-based interceptors at Ft Greely in Alaska, plus two interceptors at Vandenberg AFB, California. The U.S. plans to have 21 interceptor missiles by the end of 2007.[7] The system was initially called National Missile Defense (NMD), but in 2003 the ground-based component was renamed Ground-Based Midcourse Defense (GMD).

Defending against cruise missiles is similar to defending against hostile, low-flying manned aircraft. As with aircraft defense, countermeasures such as chaff, flares, and low altitude can complicate targeting and missile interception. High-flying radar aircraft such as AWACS can often identify low flying threats by using doppler radar. Another possible method is using specialized satellites to track these targets. By coupling a target's kinetic inputs with infrared and radar signatures it may be possible to overcome the countermeasures.

In March 2008, the U.S. Congress convened hearings to re-examine the status of missile defense in U.S. military strategy. Upon taking office, President Obama directed a comprehensive review of ballistic missile defense policy and programs. The review's findings related to Europe were announced on 17 September 2009. The full Ballistic Missile Defense Review (BMDR) Report was published in February 2010.[8][9]

The Conference of National Armaments Directors (CNAD) is the senior NATO committee which acts as the tasking authority for the theater missile defense program. The ALTBMD Program Management Organization, which comprises a Steering Committee and a Program Office hosted by the NATO C3 Agency, directs the program and reports to the CNAD. The focal point for consultation on full-scale missile defense is the Reinforced Executive Working Group. The CNAD is responsible for conducting technical studies and reporting the outcome to the Group. The NRC Ad hoc Working Group on TMD is the steering body for NATO-Russia cooperation on theater missile defense.

By early 2010, NATO will have an initial capability to protect Alliance forces against missile threats and is examining options for protecting territory and populations. This is in response to the proliferation of weapons of mass destruction and their delivery systems, including missiles of all ranges. The Alliance is conducting three missile defense related activities:

By early 2010, the Alliance will have an interim capability to protect troops in a specific area against short-range and medium-range ballistic missiles (up to 3,000 kilometers).

The end system will consist of a multi-layered system of systems, comprising low and high-altitude defenses (also called lower and upper layer defenses), including Battle Management Command, Control, Communications and Intelligence (BMC3I), early warning sensors, radar, and various interceptors. NATO member countries will provide the sensors and weapon systems, while NATO will develop the BMC3I segment and facilitate the integration of all these elements.

A Missile Defense Feasibility Study was launched after the 2002 Prague Summit. The NATO Consultation, Command and Control Agency (NC3A) and NATO’s Conference of National Armaments Directors (CNAD) were also involved in negotiations. The study concluded that missile defense is technically feasible, and it provided a technical basis for ongoing political and military discussions regarding the desirability of a NATO missile defense system.

During the 2008 Bucharest Summit, the alliance discussed the technical details as well as the political and military implications of the proposed elements of the US missile defense system in Europe. Allied leaders recognized that the planned deployment of European-based US missile defense assets would help protect North American Allies, and agreed that this capability should be an integral part of any future NATO-wide missile defense architecture. However, these opinions are in the process of being reconstructed given the Obama Administration’s decision in 2009 to replace the long range interceptor project in Poland with a short/medium range interceptor.

Russian Foreign Minister Sergei Lavrov has stated that NATOs pattern of deployment of Patriot missiles indicates that these will be used to defend against Iranian missiles in addition to the stated goal of defending against spillover from the Syrian civil war.[10]

Under the auspices of the NATO-Russia Council, a study was launched in 2003 to assess possible levels of interoperability among in-theater missile defense systems of NATO Allies and Russia.

Together with the interoperability study, several computer-assisted exercises have been held to provide the basis for future improvements to interoperability and to develop mechanisms and procedures for joint operations in the area of in-theater missile defense.[11]

In 2012 the system will achieve an "interim capability" that will for the first time offer American forces in Europe some protection against IRBM attack.[13] However these interceptors may be poorly placed and of the wrong type to defend the United States, in addition to American troops and facilities in Europe.[14]

According to BioPrepWatch, NATO has signed a 136 million euro contract with ThalesRaytheonSystems to upgrade its current theatre missile defense program.

The project, called ACCS Theatre Missile Defence 1, will bring new capabilities to NATO’s Air Command and Control System, including updates for processing ballistic missile tracks, additional satellite and radar feeds, improvements to data communication and correlation features. The upgrade to its theatre missile defense command and control system will allow for the Alliance to connect national sensors and interceptors in defense against short and medium-range ballistic missiles. According to NATO’s Assistant Secretary General for Defense Investment Patrick Auroy, the execution of this contract will be a major technical milestone forward for NATO’s theatre missile defense. The project is expected to be complete by 2015.[15]